If piracy is theft then yes, it would be a theft. It could be argued that piracy is not theft, but that's a separate argument.

All three of your premises appear faulty. First off, if the author entered into an agreement with the publisher then the publisher has rights regardless of whether or not the publisher had anything to do with authorship or compilation of the book. Second, I am pretty sure that it is possible to commit an injustice against the deceased. This happens whenever you commit an injustice against his estate (heirs) who are an extension of his estate. Third, injustice is unjust not because somebody is harmed but because it is unjust. The fact that somebody is being harmed is bad, but it's not the essence of the theft. You can commit a theft even without doing any harm (e.g. as when somebody steals a couple of dollars from a rich man, or when somebody "borrows" an item without permission and then returns it before anyone ever finds out). So the argument "the heirs aren't profiting from it anyway" does not really hold.

Again, I do think it's probably ok to pirate the book on the grounds that piracy is not theft, but that's a separate argument. And I'm not entirely sure what to think. In most cases piracy does not seem to be theft but in some cases it does feels like a theft. Not sure if this feeling is because I've been indoctrinated or if it's a legitimate feeling. If the latter, then piracy might really be theft. In which case it might not be ok.